Raw fuel gas produced by most commercial fuel gasifiers and gasifiers now under development contains various concentrations of coal tar, polycyclic aromatic hydrocarbons, and soot. These can cause serious operational problems in heat recovery and gas cleaning, but more importantly, they represent a serious environmental hazard. Many of the polycyclic aromatic compounds found in raw synthetic fuel gases are either direct or latent carcinogens.
The current approach to removing these compounds from the fuel gas involves adding gas cleaning systems to the coal gasifiers to remove the contaminants present in the fuel gas, including coal tar, polycyclic aromatic hydrocarbons, and soot. There are two types of gas cleaning systems currently in use or under consideration. In "cold gas cleaning," the raw fuel gas is cooled either by direct contact with water in a spray tower or in a scrubber, or by heat exchanger with the clean fuel gas in a high temperature heat exchanger. After cooling, the gas is cleaned to remove tar, polycyclic aromatic hydrocarbons, particulates, sulfur compounds, ammonia, and trace contaminants. In "hot gas cleaning," an attempt is made to remove particulate matter, sulfur compounds (e.g., H.sub.2 S, COS), and trace contaminants (e.g., NH.sub.3, alkali metals, etc.), at high temperature (e.g. about 1600.degree. F.).
In cold gas cleaning, coal tar and polycyclic aromatic hydrocarbons are condensed on particulate matter and enter waste water streams. If coal gasifiers employing "cold gas" cleaning systems are operated on a large scale, huge quantities of solid wastes and waste water, contaminated by polycyclic aromatic hydrocarbons will be generated. The safe disposal of these wastes constitutes an environmental problem of major proportion.
Because of their remarkable thermal stability, only a relatively small portion of the polycyclic aromatic hydrocarbons are decomposed in "hot" gas cleaning reactors. Under the conditions encountered in most coal gasification processes the free radicals formed during thermal decomposition of the polycyclic aromatic hydrocarbons repolymerize, forming higher molecular weight polycyclic aromatic hydrocarbons and soot.
These polycyclic aromatic compounds and soot will be burned together with the fuel gas in gas turbine combustors, power plant boilers, or industrial burners. Because polycyclic aromatic hydrocarbons resist complete combustion, some polycyclic aromatics, (though a smaller quantity than in systems using cold gas clean-up,) will be released into the atmosphere with the combustion products. These polycyclic aromatic hydrocarbons will condense on particulate matter in the air and will be breathed by people and animals. Eventually, these compounds will settle on the ground, water bodies, and plant life. Thus, neither of these two methods currently in use or under consideration represents a satisfactory long-term solution to the problem of polycyclic aromatic hydrocarbons in coal gasification.
The quantity of polycyclic aromatic hydrocarbons generated by coal gasifiers depends upon the temperature level at which the coal gasifiers are operating and decreases with increasing temperature. Although it is tempting to try to reduce the quantities of polycyclic aromatics released into the environment by operating coal gasifiers at high temperatures, this approach presents some new problems. High temperature gasifiers have substantially lower thermal ("cold gas") efficiencies than coal gasifiers operating at lower temperatures (because more carbon has to be burned to maintain the high temperature). Also, experience shows that coal ash and particulate matter from even the highest temperature gasifiers, contain significant amounts of polycyclic aromatic hydrocarbons.
To improve the efficiency of the use of coal resources and to reduce contamination of the environment, it is necessary to develop means to reduce the emissions of polycyclic aromatic hydrocarbons in coal gasifiers, irrespective of the temperature levels at which these gasifiers operate.